Fix creating missing mountpoints in linux-user-chroot backend, update docs

Creating missing mountpoints is done later now, so that there's less
chance of them being created if bad sandbox configuration was passed.
The previous code didn't seem work correctly, probably because of
Python os.path.join() having the annoying behaviour of deleting all
previous path components if it finds one with a preceeding '/'.

2 files changed, 21 insertions, 7 deletions

diff --git a/sandboxlib/__init__.py b/sandboxlib/__init__.py
index b4199f1..946ba52 100644
--- a/sandboxlib/__init__.py
+++ b/sandboxlib/__init__.py
@@ -96,8 +96,12 @@ def run_sandbox(command, cwd=None, env=None,
       - mounts: configures mount sharing. Defaults to 'undefined', where no
             no attempt is made to isolate mounts. Backends may support
             'isolated' as well.
-      - extra_mounts: a list of locations to mount inside 'rootfs_path', with
-            type and options specified in a backend-specific way.
+      - extra_mounts: a list of locations to mount inside 'rootfs_path',
+            specified as a list of tuples of (source_path, target_path, type,
+            options). The 'type' and 'options' should match what would be
+            specified in /etc/fstab, but a backends may support only a limited
+            subset of values. The 'target_path' is relative to filesystem_root
+            and will be created before mounting if it doesn't exist.
       - network: configures network sharing. Defaults to 'undefined', where
             no attempt is made to either prevent or provide networking
             inside the sandbox. Backends may support 'isolated' and/or other
diff --git a/sandboxlib/linux_user_chroot.py b/sandboxlib/linux_user_chroot.py
index cca734f..c397d8f 100644
--- a/sandboxlib/linux_user_chroot.py
+++ b/sandboxlib/linux_user_chroot.py
@@ -65,8 +65,6 @@ def process_mount_config(root, mounts, extra_mounts):
         "'linux-user-chroot' backend. Supported values: %s" \
         % (mounts, ', '.join(supported_values))
 
-    extra_mounts = sandboxlib.validate_extra_mounts(extra_mounts)
-
     # Use 'unshare' to create a new mount namespace.
     #
     # In order to mount the things specified in 'extra_mounts' inside the
@@ -117,9 +115,6 @@ def process_mount_config(root, mounts, extra_mounts):
     ''')
 
     for source, mount_point, mount_type, mount_options in extra_mounts:
-        path = os.path.join(root, mount_point)
-        if not os.path.exists(path):
-            os.makedirs(path)
         mount_script_args.extend((mount_point, mount_type, source,
                                   mount_options))
     mount_script_args.append('--')
@@ -259,6 +254,17 @@ def process_writable_paths(fs_root, writable_paths):
     return extra_linux_user_chroot_args
 
 
+def create_mount_points_if_missing(filesystem_root, mount_info_list):
+    for source, mount_point, mount_type, mount_options in mount_info_list:
+        # Strip the preceeding '/' from mount_point, because it'll break
+        # os.path.join().
+        mount_point_no_slash = os.path.relpath(mount_point, start='/')
+
+        path = os.path.join(filesystem_root, mount_point_no_slash)
+        if not os.path.exists(path):
+            os.makedirs(path)
+
+
 def run_sandbox(command, cwd=None, env=None,
                 filesystem_root='/', filesystem_writable_paths='all',
                 mounts='undefined', extra_mounts=None,
@@ -269,6 +275,8 @@ def run_sandbox(command, cwd=None, env=None,
 
     linux_user_chroot_command = ['linux-user-chroot']
 
+    extra_mounts = sandboxlib.validate_extra_mounts(extra_mounts)
+
     unshare_command = process_mount_config(
         root=filesystem_root, mounts=mounts, extra_mounts=extra_mounts or [])
 
@@ -282,6 +290,8 @@ def run_sandbox(command, cwd=None, env=None,
 
     linux_user_chroot_command.append(filesystem_root)
 
+    create_mount_points_if_missing(filesystem_root, extra_mounts)
+
     argv = (unshare_command + linux_user_chroot_command + command)
     exit, out, err = sandboxlib._run_command(argv, stdout, stderr, env=env)
     return exit, out, err